Method and apparatus for atrial arrhythmia episode detection

ABSTRACT

Techniques and devices for implementing the techniques for adjusting atrial arrhythmia detection based on analysis of one or more P-wave sensing windows associated with one or more R-waves. An implantable medical device may determine signal characteristics of the cardiac signal within the P-wave sensing window, determine whether the cardiac signal within the sensing window corresponds to a P-wave based on the determined signal characteristics, determine a signal to noise ratio of the cardiac signal within the sensing window, update the arrhythmia score when the P-wave is identified in the sensing window and the determined signal to noise ratio satisfies a signal to noise threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/700,087 (published as U.S. Publication No. 2020/0100694), filed Dec.2, 2019, which is a continuation of U.S. patent application Ser. No.15/551,924 (granted as U.S. Pat. No. 10,492,706), filed Feb. 18, 2016,which is a U.S. national stage entry of International Application No.PCT/US2016/018408, filed Feb. 18, 2016, which claims the benefit of andpriority from U.S. patent application Ser. No. 14/695,111 (granted asU.S. Pat. No. 9,603,543), filed Apr. 24, 2015 and U.S. ProvisionalApplication Ser. No. 62/117,785, filed Feb. 18, 2015, the entirecontents of each of which are incorporated herein by reference.

TECHNICAL FIELD

The disclosure relates generally to implantable cardiac medical devicesand, in particular, to a method for and apparatus for detecting atrialtachyarrhythmia episodes in an implantable cardiac medical device.

BACKGROUND

During normal sinus rhythm (NSR), the heart beat is regulated byelectrical signals produced by the sino-atrial (SA) node located in theright atrial wall. Each atrial depolarization signal produced by the SAnode spreads across the atria, causing the depolarization andcontraction of the atria, and arrives at the atrioventricular (A-V)node. The A-V node responds by propagating a ventricular depolarizationsignal through the bundle of His of the ventricular septum andthereafter to the bundle branches and the Purkinje muscle fibers of theright and left ventricles.

Atrial tachyarrhythmia includes the disorganized form of atrialfibrillation and varying degrees of organized atrial tachycardia,including atrial flutter. Atrial fibrillation (AF) occurs because ofmultiple focal triggers in the atrium or because of changes in thesubstrate of the atrium causing heterogeneities in conduction throughdifferent regions of the atria. The ectopic triggers can originateanywhere in the left or right atrium or pulmonary veins. The AV nodewill be bombarded by frequent and irregular atrial activations but willonly conduct a depolarization signal when the AV node is not refractory.The ventricular cycle lengths will be irregular and will depend on thedifferent states of refractoriness of the AV-node.

In the past, atrial arrhythmias have been largely undertreated due tothe perception that these arrhythmias are relatively benign. As moreserious consequences of persistent atrial arrhythmias have come to beunderstood, such as an associated risk of relatively more seriousventricular arrhythmias and stroke, there is a growing interest inmonitoring and treating atrial arrhythmias.

Methods for discriminating arrhythmias that are atrial in origin fromarrhythmias originating in the ventricles have been developed for use indual chamber implantable devices wherein both an atrial EGM signal and aventricular EGM signal are available. Discrimination of arrhythmias canrely on event intervals (PP intervals and RR intervals), event patterns,and EGM morphology. Such methods have been shown to reliablydiscriminate ventricular arrhythmias from supra-ventricular arrhythmias.In addition, such methods have been developed for use in single chamberimplantable devices, subcutaneous implantable devices, and externalmonitoring devices, where an adequate atrial EGM signal havingacceptable signal-to-noise ratio is not always available for use indetecting and discriminating atrial arrhythmias.

Occasionally, false detection of atrial fibrillation may occur in asubcutaneous device during runs of ectopic rhythm with irregularcoupling intervals or underlying sinus variability/sick sinus. Inaddition, false detection of atrial tachycardia may occur in asubcutaneous device during ectopy and regular normal sinus rhythm.Therefore, what is needed is a method for improving detection of atrialtachyarrhythmia to reduce false detection in a medical device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary medical device fordetecting an arrhythmia according to an embodiment of the presentdisclosure.

FIG. 2 is a functional schematic diagram of the medical device of FIG. 1.

FIG. 3 is a flowchart of a method for detecting an atrial arrhythmiaaccording to an embodiment of the disclosure.

FIG. 4 is a schematic diagram of detecting an atrial arrhythmiaaccording to an embodiment of the disclosure.

FIG. 5 is a flowchart of a method of detecting an atrial arrhythmia in amedical device according to an embodiment of the disclosure.

FIG. 6 is a schematic diagram of detecting an atrial arrhythmia in amedical device, according to an embodiment of the disclosure.

FIGS. 7A and 7B are flowcharts of a method of detecting an atrialarrhythmia in a medical device according to an embodiment of thedisclosure.

FIG. 8 is a flowchart of a method of determining an atrial arrhythmiaaccording to an embodiment of the disclosure.

FIG. 9 is a flowchart of a method of detecting an atrial arrhythmiaaccording to an embodiment of the disclosure.

DETAILED DESCRIPTION

In the following description, references are made to illustrativeembodiments for carrying out the methods described herein. It isunderstood that other embodiments may be utilized without departing fromthe scope of the disclosure.

In various embodiments, ventricular signals are used for determiningsuccessive ventricular cycle lengths for use in detecting atrialarrhythmias. The atrial arrhythmia detection methods do not require anelectrode positioned within the atrium as an atrial signal source todirectly sense the atrial signal within the heart; i.e., the device maybe a single chamber device having an electrode positioned only withinthe ventricle, or a subcutaneous device having no electrode positionedwithin the heart. The methods presented herein may be embodied insoftware, hardware or firmware in implantable or external medicaldevices. Such devices include implantable monitoring devices havingcardiac EGM/ECG monitoring capabilities and associated EGM/ECG senseelectrodes, which may be intracardiac, epicardial, or subcutaneouselectrodes.

The methods described herein can also be incorporated in implantablemedical devices having therapy delivery capabilities, such as singlechamber or bi-ventricular pacing systems or ICDs that sense the R-wavesin the ventricles and deliver an electrical stimulation therapy to theventricles. The atrial arrhythmia detection methods presently disclosedmay also be incorporated in external monitors having ECG electrodescoupled to the patient's skin to detect R-waves, e.g. Holter monitors,or within computerized systems that analyze pre-recorded ECG or EGMdata. Embodiments may further be implemented in a patient monitoringsystem, such as a centralized computer system which processes data sentto it by implantable or wearable monitoring devices, includingsubcutaneous devices having loop recorders.

FIG. 1 is a schematic diagram of an exemplary medical device fordetecting an arrhythmia according to an embodiment of the presentdisclosure. As illustrated in FIG. 1 , a medical device according to anembodiment of the present disclosure may be in the form of animplantable cardioverter defibrillator (ICD) 10 a connector block 12that receives the proximal ends of a right ventricular lead 16, a rightatrial lead 15 and a coronary sinus lead 6, used for positioningelectrodes for sensing and stimulation in three or four heart chambers.Right ventricular lead 16 is positioned such that its distal end is inthe right ventricle for sensing right ventricular cardiac signals anddelivering pacing or shocking pulses in the right ventricle. For thesepurposes, right ventricular lead 16 is equipped with a ring electrode24, an extendable helix electrode 26 mounted retractably within anelectrode head 28, and a coil electrode 20, each of which are connectedto an insulated conductor within the body of lead 16. The proximal endof the insulated conductors are coupled to corresponding connectorscarried by bifurcated connector 14 at the proximal end of lead 16 forproviding electrical connection to the ICD 10. It is understood thatalthough the device illustrated in FIG. 1 is a dual chamber device,other devices such as single chamber devices may be utilized to performthe technique of the present disclosure described herein.

The right atrial lead 15 is positioned such that its distal end is inthe vicinity of the right atrium and the superior vena cava. Lead 15 isequipped with a ring electrode 21 and an extendable helix electrode 17,mounted retractably within electrode head 19, for sensing and pacing inthe right atrium. Lead 15 is further equipped with a coil electrode 23for delivering high-energy shock therapy. The ring electrode 21, thehelix electrode 17 and the coil electrode 23 are each connected to aninsulated conductor with the body of the right atrial lead 15. Eachinsulated conductor is coupled at its proximal end to a connectorcarried by bifurcated connector 13.

The coronary sinus lead 6 is advanced within the vasculature of the leftside of the heart via the coronary sinus and great cardiac vein. Thecoronary sinus lead 6 is shown in the embodiment of FIG. 1 as having adefibrillation coil electrode 8 that may be used in combination witheither the coil electrode 20 or the coil electrode 23 for deliveringelectrical shocks for cardioversion and defibrillation therapies. Inother embodiments, coronary sinus lead 6 may also be equipped with adistal tip electrode and ring electrode for pacing and sensing functionsin the left chambers of the heart. The coil electrode 8 is coupled to aninsulated conductor within the body of lead 6, which provides connectionto the proximal connector 4.

The electrodes 17 and 21 or 24 and 26 may be used as true bipolar pairs,commonly referred to as a “tip-to-ring” configuration. Further,electrode 17 and coil electrode 20 or electrode 24 and coil electrode 23may be used as integrated bipolar pairs, commonly referred to as a“tip-to-coil” configuration. In accordance with the invention, ICD 10may, for example, adjust the electrode configuration from a tip-to-ringconfiguration, e.g., true bipolar sensing, to a tip-to-coilconfiguration, e.g., integrated bipolar sensing, upon detection ofoversensing in order to reduce the likelihood of future oversensing. Inother words, the electrode polarities can be reselected in response todetection of oversensing in an effort to reduce susceptibility ofoversensing. In some cases, electrodes 17, 21, 24, and 26 may be usedindividually in a unipolar configuration with the device housing 11serving as the indifferent electrode, commonly referred to as the “can”or “case” electrode.

The device housing 11 may also serve as a subcutaneous defibrillationelectrode in combination with one or more of the defibrillation coilelectrodes 8, 20 or 23 for defibrillation of the atria or ventricles. Itis recognized that alternate lead systems may be substituted for thethree lead system illustrated in FIG. 1 . While a particularmulti-chamber ICD and lead system is illustrated in FIG. 1 ,methodologies included in the present invention may adapted for use withany single chamber, dual chamber, or multi-chamber ICD or pacemakersystem, subcutaneous implantable device, or other internal or externalcardiac monitoring device.

ICD 10 may alternatively be configured as a subcutaneous device havingsensing or pacing electrodes incorporated on the housing 11 of thedevice in which case transvenous leads are not required. A subcutaneousdevice may be coupled to a lead tunneled subcutaneously or submuscularlyfor delivering transthoracic pacing pulses and/or sensing ECG signals.An exemplary subcutaneous device is described in commonly assigned U.S.patent application Ser. Nos. 14/604,111 and 14/604,260. The techniquesdescribed herein can also be implemented in an external device, e.g.including patch electrodes and optionally another physiological sensorif desired, that can sense variable parameters as described herein.

FIG. 2 is a functional schematic diagram of the medical device of FIG. 1. This diagram should be taken as exemplary of the type of device withwhich the invention may be embodied and not as limiting. The disclosedembodiment shown in FIG. 2 is a microprocessor-controlled device, butthe methods of the present invention may also be practiced with othertypes of devices such as those employing dedicated digital circuitry.

With regard to the electrode system illustrated in FIG. 1 , ICD 10 isprovided with a number of connection terminals for achieving electricalconnection to the leads 6, 15, and 16 and their respective electrodes. Aconnection terminal 311 provides electrical connection to the housing 11for use as the indifferent electrode during unipolar stimulation orsensing. The connection terminals 320, 313, and 318 provide electricalconnection to coil electrodes 20, 8 and 23 respectively. Each of theseconnection terminals 311, 320, 313, and 318 are coupled to the highvoltage output circuit 234 to facilitate the delivery of high energyshocking pulses to the heart using one or more of the coil electrodes 8,20, and 23 and optionally the housing 11.

The connection terminals 317 and 321 provide electrical connection tothe helix electrode 17 and the ring electrode 21 positioned in the rightatrium. The connection terminals 317 and 321 are further coupled to anatrial sense amplifier 204 for sensing atrial signals such as P-waves.The connection terminals 326 and 324 provide electrical connection tothe helix electrode 26 and the ring electrode 24 positioned in the rightventricle. The connection terminals 326 and 324 are further coupled to aventricular sense amplifier 200 for sensing ventricular signals.

The atrial sense amplifier 204 and the ventricular sense amplifier 200preferably take the form of automatic gain controlled amplifiers withadjustable sensitivity. In accordance with the invention, ICD 10 and,more specifically, microprocessor 224 automatically adjusts thesensitivity of atrial sense amplifier 204, ventricular sense amplifier200 or both in response to detection of oversensing in order to reducethe likelihood of oversensing. Ventricular sense amplifier 200 andatrial sense amplifier 204 operate in accordance with originallyprogrammed sensing parameters for a plurality of cardiac cycles, andupon detecting oversensing, automatically provides the corrective actionto avoid future oversensing. In this manner, the adjustments provided byICD 10 to amplifiers 200 and 204 to avoid future oversensing are dynamicin nature. Particularly, microprocessor 224 increases a sensitivityvalue of the amplifiers, thus reducing the sensitivity, when oversensingis detected. Atrial sense amplifier 204 and ventricular sense amplifier200 receive timing information from pacer timing and control circuitry212.

Specifically, atrial sense amplifier 204 and ventricular sense amplifier200 receive blanking period input, e.g., ABLANK and VBLANK,respectively, which indicates the amount of time the electrodes are“turned off” in order to prevent saturation due to an applied pacingpulse or defibrillation shock. As will be described, the blankingperiods of atrial sense amplifier 204 and ventricular sense amplifier200 and, in turn, the blanking periods of sensing electrodes associatedwith the respective amplifiers may be automatically adjusted by ICD 10to reduce the likelihood of oversensing. The general operation of theventricular sense amplifier 200 and the atrial sense amplifier 204 maycorrespond to that disclosed in U.S. Pat. No. 5,117,824, by Keimel, etal. Whenever a signal received by atrial sense amplifier 204 exceeds anatrial sensitivity, a signal is generated on the P-out signal line 206.Whenever a signal received by the ventricular sense amplifier 200exceeds a ventricular sensitivity, a signal is generated on the R-outsignal line 202.

Switch matrix 208 is used to select which of the available electrodesare coupled to a wide band amplifier 210 for use in digital signalanalysis. Selection of the electrodes is controlled by themicroprocessor 224 via data/address bus 218. The selected electrodeconfiguration may be varied as desired for the various sensing, pacing,cardioversion and defibrillation functions of the ICD 10. Specifically,microprocessor 224 may modify the electrode configurations based ondetection of oversensing due to cardiac or non-cardiac origins. Upondetection of R-wave oversensing, for example, microprocessor 224 maymodify the electrode configuration of the right ventricle from truebipolar sensing, e.g., tip-to-ring, to integrated bipolar sensing, e.g.,tip-to-coil.

Signals from the electrodes selected for coupling to bandpass amplifier210 are provided to multiplexer 220, and thereafter converted tomulti-bit digital signals by A/D converter 222, for storage in randomaccess memory 226 under control of direct memory access circuit 228 viadata/address bus 218. Microprocessor 224 may employ digital signalanalysis techniques to characterize the digitized signals stored inrandom access memory 226 to recognize and classify the patient's heartrhythm employing any of the numerous signal processing methodologiesknown in the art. An exemplary tachyarrhythmia recognition system isdescribed in U.S. Pat. No. 5,545,186 issued to Olson et al.

Upon detection of an arrhythmia, an episode of EGM data, along withsensed intervals and corresponding annotations of sensed events, arepreferably stored in random access memory 226. The EGM signals storedmay be sensed from programmed near-field and/or far-field sensingelectrode pairs. Typically, a near-field sensing electrode pair includesa tip electrode and a ring electrode located in the atrium or theventricle, such as electrodes 17 and 21 or electrodes 26 and 24. Afar-field sensing electrode pair includes electrodes spaced furtherapart such as any of: the defibrillation coil electrodes 8, 20 or 23with housing 11; a tip electrode 17 or 26 with housing 11; a tipelectrode 17 or 26 with a defibrillation coil electrode 20 or 23; oratrial tip electrode 17 with ventricular ring electrode 24. The use ofnear-field and far-field EGM sensing of arrhythmia episodes is describedin U.S. Pat. No. 5,193,535, issued to Bardy. Annotation of sensedevents, which may be displayed and stored with EGM data, is described inU.S. Pat. 4,374,382 issued to Markowitz.

The telemetry circuit 330 receives downlink telemetry from and sendsuplink telemetry to an external programmer, as is conventional inimplantable anti-arrhythmia devices, by means of an antenna 332. Data tobe uplinked to the programmer and control signals for the telemetrycircuit are provided by microprocessor 224 via address/data bus 218. EGMdata that has been stored upon arrhythmia detection or as triggered byother monitoring algorithms may be uplinked to an external programmerusing telemetry circuit 330. Received telemetry is provided tomicroprocessor 224 via multiplexer 220. Numerous types of telemetrysystems known in the art for use in implantable devices may be used.

The remainder of the circuitry illustrated in FIG. 2 is an exemplaryembodiment of circuitry dedicated to providing cardiac pacing,cardioversion and defibrillation therapies. The pacer timing and controlcircuitry 212 includes programmable digital counters which control thebasic time intervals associated with various single, dual ormulti-chamber pacing modes or anti-tachycardia pacing therapiesdelivered in the atria or ventricles. Pacer circuitry 212 alsodetermines the amplitude of the cardiac pacing pulses under the controlof microprocessor 224.

During pacing, escape interval counters within pacer timing and controlcircuitry 212 are reset upon sensing of R-waves or P-waves as indicatedby signals on lines 202 and 206, respectively. In accordance with theselected mode of pacing, pacing pulses are generated by atrial paceroutput circuit 214 and ventricular pacer output circuit 216. The paceroutput circuits 214 and 216 are coupled to the desired electrodes forpacing via switch matrix 208. The escape interval counters are resetupon generation of pacing pulses, and thereby control the basic timingof cardiac pacing functions, including anti-tachycardia pacing.

The durations of the escape intervals are determined by microprocessor224 via data/address bus 218. The value of the count present in theescape interval counters when reset by sensed R-waves or P-waves can beused to measure R-R intervals and P-P intervals for detecting theoccurrence of a variety of arrhythmias.

The microprocessor 224 includes associated read-only memory (ROM) inwhich stored programs controlling the operation of the microprocessor224 reside. A portion of the random access memory (RAM) 226 may beconfigured as a number of recirculating buffers capable of holding aseries of measured intervals for analysis by the microprocessor 224 forpredicting or diagnosing an arrhythmia.

In response to the detection of tachycardia, anti-tachycardia pacingtherapy can be delivered by loading a regimen from microprocessor 224into the pacer timing and control circuitry 212 according to the type oftachycardia detected. In the event that higher voltage cardioversion ordefibrillation pulses are required, microprocessor 224 activates thecardioversion and defibrillation control circuitry 230 to initiatecharging of the high voltage capacitors 246 and 248 via charging circuit236 under the control of high voltage charging control line 240. Thevoltage on the high voltage capacitors is monitored via a voltagecapacitor (VCAP) line 244, which is passed through the multiplexer 220.When the voltage reaches a predetermined value set by microprocessor224, a logic signal is generated on the capacitor full (CF) line 254,terminating charging. The defibrillation or cardioversion pulse isdelivered to the heart under the control of the pacer timing and controlcircuitry 212 by an output circuit 234 via a control bus 238. The outputcircuit 234 determines the electrodes used for delivering thecardioversion or defibrillation pulse and the pulse wave shape.

In one embodiment, the ICD 10 may be equipped with a patientnotification system 150. Any patient notification method known in theart may be used such as generating perceivable twitch stimulation or anaudible sound. A patient notification system may include an audiotransducer that emits audible sounds including voiced statements ormusical tones stored in analog memory and correlated to a programming orinterrogation operating algorithm or to a warning trigger event asgenerally described in U.S. Pat. No. 6,067,473 issued to Greeninger etal.

FIG. 3 is a flowchart of a method for detecting an atrial arrhythmiaaccording to an embodiment of the disclosure. As illustrated in FIG. 3 ,in order to determine whether a sensed cardiac signal is an atrialtachycardia event, the device determines whether the cardiac signalcontains a P-wave portion, the results of which are utilized to augmentan atrial tachycardia determination process. For example, thedetermination as to whether a P-wave is detected may be utilized toaugment detection of atrial arrhythmias based on the irregularity ofventricular cycles having RR intervals that exhibit discriminatorysignatures when plotted in a Lorenz scatter plot, such as is generallydisclosed by Ritscher et al. in U.S. Pat. No. 7,031,765, or in U.S. Pat.No. 8,639,316 to Sarkar. Other atrial arrhythmia determination methodsare generally disclosed by Sarkar, et al. in U.S. Pat. No. 7,623,911 andin U.S. Pat. No. 7,537,569, and by Houben in U.S. Pat. No. 7,627,368.

According to one embodiment, for example, during determination of signalcharacteristics for augmenting atrial tachycardia detection, the devicesenses the cardiac signal and identifies R-waves in response to thesensed cardiac signal using any known cardiac signal sensing anddetection scheme, such as that disclosed in U.S. Pat. No. 5,117,824, byKeimel, et al., for example. Upon detection of an R-wave associated withthe sensed cardiac signal, Block 300, the device determines whether theR-wave satisfies one or more RR-interval parameters, Block 302,described below. If the RR-interval parameter or parameters are notsatisfied, No in Block 302, the device waits for the next sensed R-wave,Block 300 and the process Block 300-302 is repeated using the nextR-wave. If the RR-interval parameter or parameters are satisfied, Yes inBlock 302, the device determines a P-wave window associated with theR-wave, Block 304, as described below.

Upon determination of the P-wave window, the device determines whether apredetermined number of R-waves have been identified, Block 306. Thepredetermined number of R-waves required to satisfy the determination inBlock 306 may be set as one or more R-waves, and according to oneembodiment is set as four R-waves for example. If the predeterminednumber of R-waves have not been identified and therefore a next R-waveis needed, Yes in Block 306, the device waits for the next sensedR-wave, Block 300 and the process Block 300-306 is repeated using thenext R-wave. If the predetermined number of R-waves have been identifiedand therefore a next R-wave is not needed, No in Block 306, the devicedetermines P-wave evidence, Block 308, described below, and utilizes thedetermined P-wave evidence to augment atrial arrhythmia detection, Block310, as described below.

FIG. 4 is a schematic diagram of detecting an atrial arrhythmiaaccording to an embodiment of the disclosure. As illustrated in FIGS. 3and 4 , in order to determine whether a sensed R-wave 320 satisfies theRR-interval parameters in Block 302, the device determines whether an RRinterval 322 extending between the current R-wave 320 and a previoussensed R-wave 324 is greater than an interval threshold, such as 780 msfor example. If the RR interval 322 is not greater than the intervalthreshold, the RR-interval parameter is not satisfied, No in Block 302,and the process is repeated with the next RR interval 326. If the RRinterval 322 is greater than the interval threshold, the RR intervalparameter is satisfied, Yes in Block 302.

According to another embodiment, additional RR interval parameters mayalso be included in the determination as to whether the RR intervalparameters have been satisfied in Block 302. For example, using R wave326 as an example, in addition to the determination of whether theassociated RR interval 340 satisfies the RR interval threshold, thedevice may also compare the RR interval 340 associated with the currentR wave 326 with one or more previously determined RR intervals, such asinterval 322 for example, and determine whether a relative changeassociated with the current RR-interval 340 is greater than a changethreshold, such as 100 ms, for example. If the relative changeassociated with the current RR-interval is not greater than the changethreshold, the RR interval parameter is not satisfied in Block 302. Ifthe relative change associated with the current RR interval is greaterthan the change threshold, the RR-interval parameter is satisfied inBlock 302.

In this way, if one of the RR intervals parameters are not satisfied, noP-wave window determination is made, and the process is repeated withthe next R wave. If the RR interval parameter or one of the RR intervalparameters are satisfied, the RR interval parameter is satisfied inBlock 302, and the device determines a P wave window 328 associated withthe R-wave 320 for determining whether the R wave 320 includes anassociated P-wave. For example, in order to determine the P wave window328, the device determines a P-wave window start point 330 located apredetermined distance 332 prior to the R-wave, such as 620 ms forexample, and a P wave window endpoint 334 is located at a predetermineddistance 336 subsequent to the P wave start point 330, such as 600 ms,for example, so that the P wave window 328 extends 600 ms between the Pwave start point 330 and the P wave endpoint 334. Each time a P wavewindow 328 is determined, a P wave counter is updated by one, until thepredetermined number of P wave windows are identified, such as four Pwave windows, for example.

FIG. 5 is a flowchart of a method of detecting an atrial arrhythmia in amedical device according to an embodiment of the disclosure. In responseto the predetermined number of P-waves being identified, No in Block 306of FIG. 3 , the device determines P-wave evidence for determiningwhether a P-wave is likely detected, Block 308, and utilizes thedetermined P-wave evidence to augment atrial arrhythmia detection, Block310, described below. As illustrated in FIG. 5 , during thedetermination of P-wave evidence, the device determines a characteristicP-wave in response to the current determined P-waves, Block 360. Forexample, according to one embodiment, the device determines an averageP-wave from the four determined P-waves that is identified as thecharacteristic P-wave. The associated P-wave window is then divided intoa baseline potion, Block 362, and a P-wave portion, Block 364, anddetermines signal characteristics, Block 366, for one or both of thebaseline window and the P-wave window. A determination is then made,based on the determined signal characteristics, whether thecharacteristic P-wave is confirmed as being a P-wave, Block 368.

If the characteristic P-wave is not confirmed as being a P-wave, No inBlock 368, the device waits for the next predetermined number of P-wavesto be identified, Yes in Block 306 of FIG. 3 , and the process, Blocks360-368, is repeated using the next identified P-waves. If thecharacteristic P-wave is confirmed as being a P-wave, Yes in Block 368,the device utilizes the determination of a P-wave being present toaugment atrial arrhythmia detection, Block 370, as described below.

FIG. 6 is a schematic diagram of detecting an atrial arrhythmia in amedical device, according to an embodiment of the disclosure. Asillustrated in FIGS. 5 and 6 , in order to determine P-wave evidence(Block 308 of FIG. 3 ), the device determines a characteristic P-wave400 having a characteristic P wave window 402 determined by averagingthe determined four P-wave windows, as described above. The devicedivides the P-wave window 402 into a baseline portion 404, extendingfrom the P-wave window start point 406 to a midpoint of the window 408,and a P-wave portion 410, extending from the midpoint of the window 408to a P-wave window endpoint 412. The device determines a firstderivative of the P-wave signal 414 and a second derivative of thep-wave signal 416, and determines corresponding second derivative values420 associated with positive going zero crossings 418 of the firstderivative signal 414 within the baseline portion 404 of the firstderivative signal window 402. In one embodiment, the first derivative ofthe P wave signal can be computed as the difference between pointsseparated by eight samples, and the second derivative can be computed asthe difference between points separated by four sample in the firstderivative.

The device determines the maximum amplitude of the second derivativevalues 420 associated with the positive going zero crossings 418, andthe determined maximum amplitude value is then used to generate a firstthreshold 422 for evaluating the second derivative P-wave signal 416within the P-wave portion 410 of the second derivative window 402.According to one embodiment, the threshold 422 is set as a multiple ofthe maximum of the second derivative values 420, such as twice themaximum of the second derivative values 420, for example.

In the same way, the device determines a corresponding second derivativevalue 426 for each negative going zero crossing 424 of the derivativesignal 414 within the baseline portion 404 of the window 402. A minimumamplitude of the second derivative values 426 associated with thenegative going first derivative zero crossings 424 is determined, andthe determined minimum amplitude value is used to generate a secondthreshold 428 for evaluating the second derivative P-wave signal 416within the P-wave portion 410 of the window 402. According to oneembodiment, the threshold 428 is set as a multiple of the minimum of thesecond derivative values 426, such as twice the minimum of the secondderivative values 426, for example.

Using the first threshold 422 determined in response to the determinedmaximum of the second derivative values 420, the device determines, foreach positive going zero crossing 430 of the first derivative signalwithin the P-wave portion 410 of the first derivative window, acorresponding amplitude 432 of the second derivative signal within theP-wave portion 410 of the corresponding second derivative signal 416.The device compares the resulting maximum amplitudes 432 of the secondderivative signal 416 signal within the P-wave portion 410 of the window402 to the first threshold 422. Similarly, using the second threshold422 determined in response to the determined minimum of the secondderivative values 420, the device compares, for one or more negativegoing zero crossing 434 of the first derivative signal 414, thecorresponding minimum amplitude 436 of the second derivative signal 416signal within the P-wave portion 410 of the window 402 to the secondthreshold 428.

A P-wave is determined to have occurred, Yes in Block 368 of FIG. 5 , ifeither the number of maximum amplitudes 432 determined to be greaterthan or equal to the first threshold 422 is equal to one, or the numberof minimum amplitudes 432 determined to be less than or equal to thesecond threshold 428 is equal to one. If both the number of maximumamplitudes 432 determined to be greater than or equal to the firstthreshold 422 and the number of minimum amplitudes 432 determined to beless than or equal to the second threshold 428 is not equal to one, aP-wave is not determined to have occurred, No in Block 368 of FIG. 5 .The result of the determination of whether a P-wave is identified isthen used during the determination of an atrial arrhythmia event, asdescribed below.

FIGS. 7A and 7B are flowcharts of a method of detecting an atrialarrhythmia in a medical device according to an embodiment of thedisclosure. As illustrated in FIGS. 6, 7A and 7B, during detection ofP-wave evidence, the device may also determine that the event isassociated with other atrial events, such as an atrial flutter event,for example. During determination of signal characteristics (Block 366of FIG. 5 ), the device may also determine an atrial flutter event isoccurring in response to any one of a predetermined conditions beingmet. For example, in order to identify the event as an atrial flutterevent, the device may evaluate the first derivative signal 414 and thesecond derivative signal 418 using the following Equation F1:

bwinZCpThr+bwinZCnThr=3 ANDabs{bwinZCpThr+bwinZCnThr−(pwinZCpThr+pwinZCnThr)}≤1

where bwinZCpThr is the number of the second derivative values 440associated with each of the positive going zero crossings 418 of thefirst derivative signal 414 within the baseline portion 404 of thewindow 402 having an amplitude greater than a predetermined thresholdamplitude 442, bwinZCnThr is the number of the second derivative values444 associated with each of the negative going zero crossings 424 of thefirst derivative signal 414 having an amplitude less than apredetermined threshold amplitude 446, pwinZCpThr is the number of thesecond derivative values 448 associated with each of the positive goingzero crossings 430 of the first derivative signal 414 of the P-waveportion 410 of the window 402 having an amplitude greater than apredetermined threshold amplitude 442, and pwinZCnThr is the number ofthe second derivative values 444 associated with each of the negativegoing zero crossings 424 of the first derivative signal 414 having anamplitude less than a predetermined threshold amplitude 446.

In this way, in order to identify a flutter event, the device maydetermine both the number of the second derivative values 440 associatedwith each of the positive going zero crossings 418 of the firstderivative signal 414 within the baseline portion 404 of the window 402having an amplitude greater than a predetermined threshold amplitude 442(pwinZCpThr), and the number of the second derivative values 444associated with each of the negative going zero crossings 424 of thefirst derivative signal 414 having an amplitude less than apredetermined threshold amplitude 446 (pwinZCnThr), Block 500. Forexample, according to one embodiment, the amplitude thresholds 442 and446 may be set as 8 microvolts for the positive going zero crossings 418and as −8 microvolts for the negative going zero crossings 424.

The device determines whether an amplitude threshold is satisfied inresponse to the determined number of amplitudes 440 and 444 exceedingthe thresholds 442 and 446, Block 502, by determining whether a total ofthe number of amplitudes 440 and 440 is equal to a predetermined numberamplitudes, such as three amplitudes, for example. If the number ofamplitudes exceeding the thresholds 442 and 446 (bwinZCpThr+bwinZCnThr)is satisfied, Yes in Block 502, the device determines both the number ofthe second derivative values 448 associated with each of the positivegoing zero crossings 430 of the first derivative signal 414 of theP-wave portion 410 of the window 402 having an amplitude greater thanthe threshold amplitude 442 (pwinZCpThr), and the number of the secondderivative values 450 associated with each of the negative going zerocrossings 434 of the first derivative signal 414 having an amplitudeless than the threshold amplitude 446 (pwinZCnThr), Block 504. Arelative baseline portion 404 and P-wave portion 420 amplitudedifference (abs {bwinZCpThr+bwinZCnThr−(pwinZCpThr+pwinZCnThr)}) isdetermined using the determined second derivative values 448 and 450 inBlock 504, and a determination is made as to whether the relativeamplitude difference satisfies an amplitude threshold difference, Block506. For example, the amplitude threshold difference is satisfied if therelative amplitude difference is less than or equal to a predeterminedthreshold, such as one, for example. In this way, a flutter event may beidentified in response to both the baseline portion 404 amplitudedifference threshold being satisfied (bwinZCpThr+bwinZCnThr=3), Yes inBlock 502, and the determined relative amplitude difference (abs{bwinZCpThr+bwinZCnThr−(pwinZCpThr+pwinZCnThr)}≤1) being satisfied.

According to another embodiment, in order to identify the event as anatrial flutter event, the device may evaluate the first derivativesignal 414 and the second derivative signal 418 using the followingEquation F2:

bwinZCpThr+bwinZCnThr=3 AND abs{bwinZCpThr+bwinZCnThr−(bwinZCp+bwinZCn)}≤1

where bwinZCp is the number of positive zero crossings 418 within thebaseline portion 404 of the window 402 and bwinZCn is the number ofnegative zero crossings 424 within the baseline portion 404 of thewindow 402, and the remaining variables are as described above inEquation F1.

In this way, if the amplitude difference is not satisfied, No in Block506, in addition to the amplitude threshold variables (bwinZCpThr) and(bwinZCnThr), the device determines both the number of positive zerocrossings 418 within the baseline portion 404 of the window 402(bwinZCp) and the number of negative zero crossings 424 within thebaseline portion 404 of the window 402 (bwinZCn) and determines whetheran amplitude/zero crossing threshold is satisfied, Block 512, inresponse to the amplitude threshold variables (bwinZCpThr) and(bwinZCnThr), the number of positive zero crossings 418 within thebaseline portion 404 of the window 402 (bwinZCp) and the number ofnegative zero crossings 424 within the baseline portion 404(bwinZCnThr). For example, the amplitude/zero crossing threshold issatisfied, Yes in Block 512, and therefore a flutter event is occurring,Block 508, in response to both the amplitude threshold(bwinZCpThr+bwinZCnThr=3) and an amplitude/zero crossing threshold (abs{bwinZCpThr+bwinZCnThr−(bwinZCp+bwinZCn)}≤1) being satisfied.

According to another embodiment, in order to identify the event as anatrial flutter event, the device may evaluate the first derivativesignal 414 and the second derivative signal 418 using the followingEquation F3:

bwinZCpThr+pwinZCpThr=3 AND abs{bwinZCpThr+pwinZCpThr−(bwinZCp+pwinZCp)}≤1

where pwinZCp is the number of positive going zero crossings 430 withinthe P-wave portion 410 of the window 402, and the remaining variablesare as described above in Equations F1 and F2. In this way, if eitherthe number of amplitudes exceeding the thresholds 442 and 446(bwinZCpThr+bwinZCnThr) is not satisfied, No in Block 502 or theamplitude/zero crossing threshold is not satisfied, No in Block 512, inaddition to the necessary previously described variables, Blocks 500,504, and 510, the device determines the number of positive going zerocrossings 430 within the P-wave portion 410 of the window 402, Block514. The device then determines whether an amplitude threshold issatisfied, Block 516, by determining the sum of the number of the secondderivative values 440 associated with each of the positive going zerocrossings 418 of the first derivative signal 414 within the baselineportion 404 of the window 402 having an amplitude greater than apredetermined threshold amplitude 442 (bwinZCpThr) and the number of thesecond derivative values 448 associated with each of the positive goingzero crossings 430 of the first derivative signal 414 of the P-waveportion 410 of the window 402 having an amplitude greater than apredetermined threshold amplitude 442 (pwinZCpThr) satisfies anamplitude threshold, Block 516, such as being equal to 3, for example,(bwinZCpThr+pwinZCpThr=3).

If the amplitude threshold is satisfied, Yes in Block 516, determines asum (bwinZCp+pwinZCp) of the number of positive crossing points 418 inthe baseline portion 404 (bwinZCp) and the number of positive crossings434 in the P-wave portion 410 (pwinZCp), Block 518. A determination isthen made as to whether an amplitude/zero crossing difference has beensatisfied, Block 520, and if the difference has been satisfied, Yes inBlock 520, a flutter event is identified, Block 508. According to oneembodiment, for example, in order to determine whether theamplitude/zero crossing difference has been satisfied in Block 520, thedevice determines whether the absolute value of the difference betweenthe sum of the number of the second derivative values 440 associatedwith each of the positive going zero crossings 418 of the firstderivative signal 414 within the baseline portion 404 of the window 402having an amplitude greater than a predetermined threshold amplitude 442(bwinZCpThr) and the number of the second derivative values 444associated with each of the negative going zero crossings 424 of thefirst derivative signal 414 having an amplitude less than apredetermined threshold amplitude 446 (pwinZCpThr) and the sum of thenumber of positive zero crossings 418 within the baseline portion 404 ofthe window 402 (bwinZCp) and is the number of positive going zerocrossings 430 within the P-wave portion 410 of the window 402 (pwinZCp)is less than a predetermined threshold, such as one for example.

According to another embodiment, in order to identify the event as anatrial flutter event, the device may evaluate the first derivativesignal 414 and the second derivative signal 418 using the followingEquation F4:

bwinZCnThr+pwinZCnThr=3 AND abs{bwinZCnThr+pwinZCnThr−(bwinZCn+pwinZCn)}≤1

where bwinZCn is the number of negative zero crossings 424 of the firstderivative signal 414 within the baseline portion 404 of the window 402,pwinZCn is the number of negative going zero crossings of the firstderivative signal 414 within the P-wave portion 410 of the window 402,and the remaining variables are as described above in Equations F1, F2and F3.

In this way, if either the amplitude threshold is not satisfied, No inBlock 516, or amplitude/zero crossing difference has been satisfied, Noin Block 520, the device determines the number of the second derivativevalues 444 associated with each of the negative going zero crossings 424of the first derivative signal 414 having an amplitude less than apredetermined threshold amplitude 446 (bwinZCnThr) and the number of thesecond derivative values 444 associated with each of the negative goingzero crossings 424 of the first derivative signal 414 having anamplitude less than a predetermined threshold amplitude 446(pwinZCnThr), Block 522, and determines whether an amplitude thresholdis satisfied, Block 524 by determining whether a sum of the number ofthe second derivative values 444 associated with each of the negativegoing zero crossings 424 of the first derivative signal 414 having anamplitude less than a predetermined threshold amplitude 446 (bwinZCnThr)and the number of the second derivative values 444 associated with eachof the negative going zero crossings 424 of the first derivative signal414 having an amplitude less than a predetermined threshold amplitude446 (bwinZCnThr+pwinZCnThr) is equal to a predetermined threshold, suchas three for example.

If the amplitude threshold is satisfied, Yes in Block 524, the devicedetermines the number of negative zero crossings 424 within the baselineportion 404 of the window 402 (bwinZCn) and the number of positive goingzero crossings 430 within the P-wave portion 410 of the window 402(pwinZCp), Block 562, and determines whether an amplitude/zero crossingthreshold is satisfied, Block 528. For example, the device determineswhether an absolute value of whether the difference between the sum ofthe number of the number of the second derivative values 444 associatedwith each of the negative going zero crossings 424 of the firstderivative signal 414 within the baseline portion 404 of the window 402having an amplitude less than the predetermined threshold amplitude 446(bwinZCnThr) and the number of the second derivative values 444associated with each of the negative going zero crossings 424 of thefirst derivative signal 414 within the P-wave portion 410 of the window402 having an amplitude less than the predetermined threshold amplitude446 (pwinZCnThr), and the sum the number of negative zero crossings 424of the first derivative signal 414 within the baseline portion 404 ofthe window 402 (of bwinZCn) and, the number of negative going zerocrossings of the first derivative signal 414 within the P-wave portion410 of the window 402 (pwinZCn) is less than or equal to a predeterminedthreshold, such as one, for example. If amplitude/zero crossingthreshold is satisfied, Yes in Block 528, a flutter event is identified,Block 508.

According to another embodiment, in order to identify the event as anatrial flutter event, the device may evaluate the first derivativesignal 414 and the second derivative signal 418 using the followingEquation F5:

bwinZCp+bwinZCn+pwinZCp+pwinZCn≥6 AND{bwinZCp+bwinZCn+pwinZCp+pwinZCn−(bwinZCpThr+bwinZCnThr+pwinZCpThr+pwinZCnThr)≤2

In this way, if either the amplitude threshold is not satisfied, No inBlock 524, or amplitude/zero crossing difference has not been satisfied,No in Block 528, the device determines a sum of the total number ofpositive zero crossings 418 and negative zero crossings 424 of the firstderivative signal 414 in the baseline portion 404 of the window 402 andthe total number of positive zero crossings 430 and negative zerocrossings 434 of the first derivative signal 414 in the P-wave portion410 of the window 402 (bwinZCp+bwinZCn+pwinZCp+pwinZCn), Block 530, anddetermines whether sum of the total positive and negative zero crossingssatisfies a zero crossing threshold, Block 534, such greater than orequal to six, for example.

If the zero crossing threshold is satisfied, Yes in Block 534, thedevice determines a difference between the sum of the total number ofpositive zero crossings 418 and negative zero crossings 424 of the firstderivative signal 414 in the baseline portion 404 of the window 402(bwinZCp+bwinZCn) and the total number of positive zero crossings 430and negative zero crossings 434 of the first derivative signal 414 inthe P-wave portion 410 of the window 402 (pwinZCp+pwinZCn) and the sumof the total number of the amplitudes 440 of the second derivativesignal 416 in the baseline portion 404 of the window 402 exceeding themaximum threshold 442 (bwinZCpThr), the total number of amplitudes 444in the baseline portion 404 of the window 402 less than the minimumthreshold 446 (bwinZCnThr), the total number of the amplitudes 448 ofthe second derivative signal 416 in the P-wave portion 410 of the window402 exceeding the maximum threshold 442 (pwinZCpThr), and the totalnumber of amplitudes 450 in the P-wave portion 410 of the window 402less than the minimum threshold 446 (pwinZCnThr), Block 536.

A determination is then made as to the whether the determined difference(bwinZCp+bwinZCn+pwinZCp+pwinZCn−(bwinZCpThr+bwinZCnThr+pwinZCpThr+pwinZCnThr)satisfies a zero crossing/amplitude difference threshold, Block 538,such as being less than or equal to 2, for example, in response to thedetermined difference. If the determined difference satisfies the zerocrossing/amplitude difference threshold, i.e.,bwinZCp+bwinZCn+pwinZCp+pwinZCn−(bwinZCpThr+bwinZCnThr+pwinZCpThr+pwinZCnThris less than or equal to 2, Yes in Block 538, a flutter event isidentified, Block 540.

According to another embodiment, in order to identify the event as anatrial flutter event, the device may evaluate the first derivativesignal 414 and the second derivative signal 418 using the followingEquation F6:

bwinPThr+bwinNThr≥8 AND bwinPThr>0 AND bwinNThr>0 AND bwinZCp+bwinZCn=3or 4

where bwinPThr is the number of amplitudes 440 of the second derivativesignal 416 within the baseline portion 404 of the window 402 that aregreater than the maximum threshold 442, and bwinNThr is the number ofamplitudes 444 of the second derivative signal 416 within the baselineportion 404 of the window 402 that are less than the minimum threshold446.

In this way, if either the zero crossing threshold is not satisfied, Noin Block 534, or the zero crossing/amplitude difference has not beensatisfied, No in Block 538, the device determines the number of secondderivative amplitudes for the baseline portion of the window thatsatisfy the amplitude threshold, Block 542, by determining the number ofamplitudes 440 of the second derivative signal 416 within the baselineportion 404 of the window 402 that are greater than the maximumthreshold 442 (bwinPThr), and the number of amplitudes 444 of the secondderivative signal 416 within the baseline portion 404 of the window 402that are less than the minimum threshold 446 (bwinNThr). The device thendetermines whether amplitude thresholds are satisfied, Block 544, bydetermining whether the determined number of second derivativeamplitudes is greater than or equal to a predetermined number ofamplitudes, such as 8 amplitudes, for example (bwinPThr+bwinNThr≥8), andwhether both the number of amplitudes 440 of the second derivativesignal 416 within the baseline portion 404 of the window 402 that aregreater than the maximum threshold 442 (bwinPThr), and the number ofamplitudes 444 of the second derivative signal 416 within the baselineportion 404 of the window 402 that are less than the minimum threshold446 (bwinNThr) are greater than a predetermined threshold number, suchas zero, for example (bwinPThr>0 AND bwinNThr>0).

If the determined number of second derivative amplitudes does notsatisfy any one of the amplitude thresholds, No in Block 544, andtherefore none of the predetermined conditions Equations F1-F6 are met,a flutter event is not identified for the current characteristic P-wave400, Block 546. If the determined number of second derivative amplitudessatisfies the amplitude thresholds, Yes in Block 544, a determination ismade as to whether the sum of the total number of positive and negativezero crossings 418 and 424 of the first derivative signal 414 within thebaseline portion 404 satisfy a predetermined baseline crossingsthreshold, Block 548, such as 3, for example (bwinZCp+bwinZCn=3 or 4).

If the baseline zero crossings threshold is satisfied, Yes in Block 548,a flutter event is identified, Block 540, and if the baseline zerocrossings threshold is not satisfied, No in Block 548, and thereforenone of the predetermined conditions Equations F1-F6 are met, a flutterevent is not identified for the current characteristic P-wave 400, Block546.

It is understood that any single one or combination and order of thepredetermined conditions F1-F6 may be utilized in determining whether aflutter event is identified, and therefore numerous combinations of theconditions F1-F6, or single ones of the conditions F1-F6 may be utilizedin determining a flutter event, and therefore the disclosure is notlimited to the combination and order of the conditions as illustrated inFIGS. 7A and 7B. In this way, a flutter event may be determined inresponse to one of any of the conditions of Equations F1-F6.

FIG. 8 is a flowchart of a method of determining an atrial arrhythmiaaccording to an embodiment of the disclosure. As illustrated in FIGS. 6and 8 , during detection of P-wave evidence, the device may alsodetermine that the event is associated with other events, such as noise,for example. During determination of signal characteristics (Block 366of FIG. 5 ), the device may also determine a noise event is occurring inresponse to any one of a predetermined conditions being met. Forexample, in order to determine whether a noise event is occurring, thedevice may determine the amplitudes of the second derivative signallocated at both the positive going zero crossing and the negative goingzero crossings of the first derivative signal 414 within the baselineportion 404 of the window 402, Block 600, and determine whether both amaximum amplitude 460 of the second derivative signal 416 at a positivezero crossing of the first derivative signal 414 within the baselineportion 404 of the window 402 and a minimum amplitude 462 of the secondderivative signal 416 at a negative zero crossing of the firstderivative signal 414 within the baseline portion 404 of the window 402satisfy a first amplitude threshold, Block 602, such as the maximumamplitude being greater than 16 microvolts and the minimum amplitudebeing less than -16 microvolts, for example.

If the first amplitude threshold is satisfied, Yes in Block 602, noiseis identified for the characteristic P-wave 400, Block 604. If the firstamplitude threshold is not satisfied, No in Block 602, the device maydetermine other conditions for indicating noise, such as determiningwhether either a maximum amplitude 460 of the second derivative signal416 at a positive zero crossing of the first derivative signal 414within the baseline portion 404 of the window 402 or a minimum amplitude462 of the second derivative signal 416 at a negative zero crossing ofthe first derivative signal 414 within the baseline portion 404 of thewindow 402 satisfy a second amplitude threshold, Block 606, such as themaximum amplitude being greater than 49 microvolts or the minimumamplitude being less than −49 microvolts, for example.

If the second amplitude threshold is satisfied, Yes in Block 606, noiseis identified for the characteristic P-wave 400, Block 604. If thesecond amplitude threshold is not satisfied, No in Block 606, the devicemay determine the number of positive going zero crossings of the firstderivative signal 414 within the baseline portion 404 of the window 402whose corresponding amplitude 460 of the second derivative signal 416 isgreater than a maximum amplitude threshold 464, such as 16 microvolts,for example, and the number of negative going zero crossings of thefirst derivative signal 414 within the baseline portion 404 of thewindow 402 whose corresponding minimum amplitude 462 of the secondderivative signal 416 is less than a minimum amplitude threshold 466,such as −16 microvolts for example, Block 608. A determination is thenmade as to whether an amplitude threshold is satisfied, Block 610, andif the amplitude threshold is satisfied, Yes in Block 610, noise isidentified, Block 604. For example, according to one embodiment, thedevice determines whether the amplitude threshold is satisfied in Block610 by determining whether a sum of both the number of positive goingzero crossings of the first derivative signal 414 within the baselineportion 404 of the window 402 whose corresponding amplitude 460 of thesecond derivative signal 416 is greater than the maximum amplitudethreshold 464 and the number of negative going zero crossings of thefirst derivative signal 414 within the baseline portion 404 of thewindow 402 whose corresponding minimum amplitude 462 of the secondderivative signal 416 is less than the minimum amplitude threshold 466being equal to a predetermined number, such as 3 for example.

If the amplitude threshold is not satisfied, No in Block 610, the devicemay determine the number of positive zero crossings within the baselineportion 404 of the window 402 and the number of negative zero crossingswithin the baseline portion 404 of the window 402, Block 612. Adetermination is made as to whether a combined amplitude threshold and abaseline crossing threshold is satisfied, Block 614, by determining, forexample, whether both the sum of the number of positive going zerocrossings of the first derivative signal 414 within the baseline portion404 of the window 402 whose corresponding amplitude 460 of the secondderivative signal 416 is greater than the maximum amplitude threshold464 and the number of negative going zero crossings of the firstderivative signal 414 within the baseline portion 404 of the window 402whose corresponding minimum amplitude 462 of the second derivativesignal 416 is less than the minimum amplitude threshold 464 is equal toa predetermined number, such as three for example, and the sum of thenumber of positive zero crossings within the baseline portion 404 of thewindow 402 and the number of negative zero crossings within the baselineportion 404 of the window 402 is within a predetermined range, such asgreater than four and less than ten, for example.

If the combined amplitude threshold and baseline crossing threshold issatisfied, Yes in Block 614, a noise event is identified, Block 604. Ifthe combined amplitude threshold and a baseline crossing threshold issatisfied, No in Block 614, the device may determine the number ofpositive going zero crossings and the number of negative going zerocrossings of the first derivative signal 414 within the P-wave portion410 of the window 302, Block 616, and determine whether a zero crossingsthreshold has been satisfied, Block 618, by determining whether a sum ofthe determined number of positive going zero crossings and the number ofnegative going zero crossings of the first derivative signal 414 isgreater than four, for example.

If the zero crossings threshold has been satisfied, Yes in Block 618, anoise event is determined, Block 604. If the zero crossings thresholdhas not been satisfied, No in Block 618, the device may determine thenumber of amplitudes 460 of the second derivative signal 416 within thebaseline portion 404 of the window 402 that are greater than the maximumthreshold 464, and the number of amplitudes 462 of the second derivativesignal 416 within the baseline portion 404 of the window 402 that areless than the minimum threshold 466, Block 620.

A determination is made as to whether a combined amplitude and baselinecrossings threshold has been satisfied, Block 622, by determining bothwhether a sum of the number of positive zero crossings within thebaseline portion 404 of the window 402 and the number of negative zerocrossings within the baseline portion 404 of the window 402 is greaterthan a baseline crossing threshold, such as four for example, andwhether a sum of the number of amplitudes 460 of the second derivativesignal 416 within the baseline portion 404 of the window 402 that aregreater than the maximum threshold 464, and the number of amplitudes 462of the second derivative signal 416 within the baseline portion 404 ofthe window 402 that are less than the minimum threshold 466 is greaterthan an amplitude threshold, such as 10 samples or 16 microvolts forexample.

If the combined amplitude and baseline crossings threshold has beensatisfied, Yes in Block 622, a noise event is determined, Block 604. Ifthe combined amplitude and baseline crossings threshold has not beensatisfied, No in Block 622, and therefore none of the predeterminedconditions, Blocks 602, 606 610, 614, 618 and 622 are met, a noise eventis not identified for the current characteristic P-wave 400, Block 624.

It is understood that any single one or combination and order of thepredetermined conditions, Blocks 602, 606 610, 614, 618 and 622, may beutilized in determining whether a noise event is identified, andtherefore numerous combinations of the conditions, or single ones of theconditions may be utilized in determining a noise event, and thereforethe disclosure is not limited to the combination and order of theconditions as illustrated in FIG. 8 . In this way, a noise event may bedetermined in response to one of any of the conditions of Blocks 602,606 610, 614, 618 and 622.

Therefore, the characteristic signal 400 may be determined to be a noiseevent if any one of the following noise conditions are met:

-   -   N1. bwinZCmax>156 ms AND bwinZCmin<−156 ms (Block 602)    -   N2. bwinZCmax>468 ms OR bwinZCmin<−468 ms (Block 606)    -   N3. bwinZCpThr+bwinZCnThr>3 (Block 610)    -   N4. bwinZCpThr+bwinZCnThr=3 AND {4<(bwinZCp+bwinZCn)<10} (Block        614)    -   N5. pwinZCp+pwinZCn>4 (Block 618)    -   N6. bwinZCp+bwinZCn>4 AND bwinPThr+bwinNThr>10 (Block 622)

where bwinZCmax is the maximum amplitude 460 of the second derivativesignal 416 at a positive zero crossing of the first derivative signal414 within the baseline portion 404 of the window 402, bwinZCmin is theminimum amplitude 462 of the second derivative signal 416 at a negativezero crossing of the first derivative signal 414 within the baselineportion 404 of the window 402, and the remaining conditions are asdescribed above.

FIG. 9 is a flowchart of a method of detecting an atrial arrhythmiaaccording to an embodiment of the disclosure. As described above, inresponse to the predetermined number of P-waves being identified, Yes inBlock 306 of FIG. 3 , the device determines P-wave evidence, Block 308,and utilizes the determined P-wave evidence to augment atrial arrhythmiadetection, Block 310, described below. As described above, according toanother embodiment, during the determination of P-wave evidence, thedevice determines a characteristic P-wave in response to the currentdetermined P-waves, Block 700. For example, according to one embodiment,the device determines an average P-wave from the four determined P-wavesthat is identified as the characteristic P-wave. The associated P-wavewindow is then divided into a baseline portion, Block 702, and a P-waveportion, Block 704, and signal characteristics are determined, Block706, for one or both of the baseline window and the P-wave window. Adetermination is then made, based on the determined signalcharacteristics, whether the characteristic P-wave is confirmed as beinga P-wave, Block 708.

If the characteristic P-wave is not confirmed as being a P-wave, No inBlock 708, the device determines whether a timer, such as a two minutetimer, for example, has expired, Block 716. If the timer has expired,P-wave evidence is used to augment an atrial arrhythmias scheme, asdescribed below, during atrial arrhythmia detection, Block 718, such asthe determination of an atrial detection score determined based on theirregularity of ventricular cycles having RR intervals that exhibitdiscriminatory signatures when plotted in a Lorenz scatter plot, forexample, which is generally disclosed by Ritscher et al. in U.S. Pat.No. 7,031,765, or in U.S. Pat. No. 8,639,316 to Sarkar. Other atrialarrhythmia determination methods that may be utilized are generallydisclosed by Sarkar, et al. in U.S. Pat. No. 7,623,911 and in U.S. Pat.No. 7,537,569, and by Houben in U.S. Pat. No. 7,627,368. If the timerhas not expired, No in Block 716, the device waits for the nextpredetermined number of P-waves to be identified, and the process,Blocks 700-708, is repeated using the next identified P-waves.

If the characteristic P-wave is confirmed as being a P-wave, Yes inBlock 708, the device determines whether a maximum signal to noise ratiois greater than a signal to noise ration SNR threshold, Block 709. Ifthe maximum signal to noise ratio is greater than the SNR threshold, Yesin Block 709, the device updates a P-Wave evidence counter, Block 714,and either repeats the process if the timer has not expired, No in Block716, or utilizes the updated P-wave evidence counter Block 714 toaugment the atrial arrhythmia detection score, Block 718, generatedbased on the irregularity of ventricular cycles, described above.

According to one embodiment, in order to determine whether a maximumsignal to noise ratio is satisfied, Block 709, the device determineswhether either the maximum amplitude 432 of the second derivative signal416 within the P-wave portion 410 of the window 402 is greater than fourtimes the maximum amplitude 420 of the second derivative signal 416within the baseline portion 404 of the window 402, or whether theminimum amplitude 436 of the second derivative signal 416 within theP-wave portion 410 of the window 402 is greater than four times theminimum amplitude 426 of the second derivative signal 416 within thebaseline portion 404 of the window 402. If either is determined tooccur, the maximum SNR threshold is satisfied, Yes in Block 709. Ifneither of the two is determined to occur, the maximum SNR threshold isnot satisfied, No in Block 709, and the device determines whether thecharacteristic P-wave 400 is identified as a flutter event, Block 710,using the flutter conditions described above. If a flutter event isidentified, Yes in Block 710, the device determines whether the timer,Block 716 as described above. If a flutter event is not identified, Noin Block 710, the device determines whether the characteristic P-wave400 is identified as a noise event, Block 712, using the conditionsdescribed above. If a noise event is identified, Yes in Block 712, thedevice determines whether the timer, Block 716 as described above. If anoise event is not identified, No in Block 712, the device updates aP-wave evidence counter, Block 714, either repeats the process if thetimer has not expired, No in Block 716, or utilizes the updated P-waveevidence counter Block 714 to augment the atrial arrhythmia detectionscore, Block 718, generated based on the irregularity of ventricularcycles, described above.

According to one embodiment, each time a P-wave is determined to occurand the P-wave evidence is updated, Block 714, the device increases aP-wave evidence counter for AF detection by two and increases an P-waveevidence counter for AT detection by one, for example, and once thetimer has expired, Yes in Block 716, the total, or a multiple of thetotal, of the respective P-wave evidence counters is used to withhold ATdetection based on regularity evidence if P-wave evidence counter for ATis greater than a threshold, 4 as an example, or are subtracted from anAF evidence score and a AT evidence score generated as described incommonly assigned U.S. Patent Publication No. 2012/0238891, to Sarkar etal., for example. The respective P-wave evidence counters can also beused to withhold the AF or AT detection respectively based on AF or ATevidence score.

In another embodiment, the amount of increase of P-wave evidencecounters for AF and AT detection in Block 714 can depend on user choiceof the mode of operation and on meeting a high SNR condition. In orderto determine whether a high signal to noise ratio is satisfied thedevice determines whether either the maximum amplitude 432 of the secondderivative signal 416 associated with a positive going zero crossing ofthe first derivative signal 414 within the P-wave portion 410 of thewindow 402 is greater than four times the maximum amplitude 420 of thesecond derivative signal 416 associated with a positive going zerocrossing of the first derivative signal 430 within the baseline portion404 of the window 402, or whether the minimum amplitude 436 of thesecond derivative signal 416 associated with a negative going zerocrossing of the first derivative signal within the P-wave portion 410 ofthe window 402 is greater than four times the minimum amplitude 426 ofthe second derivative signal 416 associated with a negative going zerocrossing of the first derivative signal within the baseline portion 404of the window 402. If either is determined to occur, the high SNRthreshold is satisfied, which will then lead to a higher increase ofP-wave evidence counters for AF and AT detection in Block 714.

Thus, an apparatus and method have been presented in the foregoingdescription with reference to specific embodiments. It is appreciatedthat various modifications to the referenced embodiments may be madewithout departing from the scope of the invention as set forth in thefollowing claims.

What is claimed is:
 1. A medical device system comprising: a medicaldevice comprising two or more electrodes, wherein the medical device isconfigured to sense a cardiac signal via the two or more electrodes; andprocessing circuitry configured to: identify, based on the cardiacsignal, a set of P-wave windows; determine, based on the cardiac signalwithin each P-wave window of the set of P-wave windows, a characteristicsignal; process the characteristic signal to identify one or more signalcharacteristics; determine, based on the one or more signalcharacteristics, whether the characteristic signal includes a P-wave;determine, based on whether the characteristic signal includes a P-wave,whether an atrial arrhythmia is detected; and generate for output, basedon the characteristic signal including the P-wave, an indication thatthe atrial arrhythmia is detected.
 2. The medical device system of claim1, wherein the processing circuitry is further configured to: identify,in the cardiac signal, one or more pairs of consecutive R-waves;determine, for each pair of consecutive R-waves of the one or more pairsof consecutive R-waves, whether an RR-interval corresponding to therespective pair of consecutive R-waves is greater than an RR-intervalthreshold; and identify, for each pair of consecutive of R-waves havingan RR-interval greater than the RR-interval threshold, a P-wave windowof the set of P-wave windows.
 3. The medical device system of claim 1,wherein to determine the characteristic signal, the processing circuitryis configured to determine an average of the cardiac signal within theset of P-wave windows.
 4. The medical device system of claim 1, whereinthe characteristic signal corresponds to a characteristic window,wherein the processing circuitry is further configured to: determine abaseline portion of the characteristic window; determine a P-waveportion of the characteristic window, wherein the baseline portion isprior to the P-wave portion, and process a first portion of thecharacteristic signal within the baseline portion of the characteristicwindow and a second portion of the characteristic signal within theP-wave portion of the characteristic window to identify the one or moresignal characteristics.
 5. The medical device system of claim 4, whereinthe baseline portion of the characteristic window extends from a startof the characteristic window to a midpoint of the characteristic window,and wherein the P-wave portion of the characteristic window extends fromthe midpoint of the characteristic window to an endpoint of thecharacteristic window.
 6. The medical device system of claim 4, whereinto determine whether the characteristic signal includes a P-wave, theprocessing circuitry is configured to: determine a second derivative ofthe characteristic signal corresponding to the P-wave portion of thecharacteristic window; identify one or more maximum amplitudes in thesecond derivative of the characteristic signal corresponding to theP-wave portion of the characteristic window; compare each maximumamplitude of the one or more maximum amplitudes to a first threshold;identify one or more minimum amplitudes in the second derivative of thecharacteristic signal corresponding to the P-wave portion of thecharacteristic window; compare each minimum amplitude of the one or moreminimum amplitudes to a second threshold; and determine that thecharacteristic signal does not include a P-wave when: a number of theone or more maximum amplitudes greater than or equal to the firstthreshold is equal to one; or a number of the one or more minimumamplitudes less than or equal to the second threshold is equal to one.7. The medical device system of claim 6, wherein the processingcircuitry is configured to determine that the characteristic signal doesnot include a P-wave when: the number of the one or more maximumamplitudes greater than or equal to the first threshold is not equal toone; and the number of the one or more minimum amplitudes less than orequal to the second threshold is not equal to one.
 8. The medical devicesystem of claim 4, wherein the processing circuitry is furtherconfigured to: determine a first derivative of the characteristic signalcorresponding to the characteristic window; determine a secondderivative of the characteristic signal corresponding to thecharacteristic window; and determine, based on the first derivative ofthe characteristic signal and the second derivative of thecharacteristic signal, whether the characteristic signal corresponds toan atrial flutter event.
 9. The medical device system of claim 8,wherein the processing circuitry is configured to: identify one or moremaximum amplitudes of the second derivative of the characteristic signalwithin the baseline portion of the characteristic window, wherein eachmaximum amplitude of the one or more maximum amplitudes is associatedwith a respective positive going zero crossing of the first derivativeof the characteristic signal; determine whether each maximum amplitudeof the one or more maximum amplitudes within the baseline portion isgreater than a first threshold amplitude; identify one or more minimumamplitudes of the second derivative of the characteristic signal withinthe baseline portion of the characteristic window, wherein each minimumamplitude of the one or more minimum amplitudes is associated with arespective negative going zero crossing of the first derivative of thecharacteristic signal; determine whether each minimum amplitude of theone or more minimum amplitudes within the baseline portion is less thana second threshold amplitude; identify one or more maximum amplitudes ofthe second derivative of the characteristic signal within the P-waveportion of the characteristic window, wherein each maximum amplitude ofthe one or more maximum amplitudes is associated with a respectivepositive going zero crossing of the first derivative of thecharacteristic signal; determine whether each maximum amplitude of theone or more maximum amplitudes within the baseline portion is greaterthan the first threshold amplitude; identify one or more minimumamplitudes of the second derivative of the characteristic signal withinthe P-wave portion of the characteristic window, wherein each minimumamplitude of the one or more minimum amplitudes is associated with arespective negative going zero crossing of the first derivative of thecharacteristic signal; and determine whether each minimum amplitude ofthe one or more minimum amplitudes within the P-wave portion is lessthan the second threshold amplitude, wherein to determine whether thecharacteristic signal corresponds to an atrial flutter event, theprocessing circuitry is configured to: calculate a first sum of a numberof the one or more maximum amplitudes within the baseline portiongreater than the first threshold amplitude and a number of the one ormore maximum amplitudes within the baseline portion greater than thesecond threshold amplitude; calculate a second sum of a number of theone or more maximum amplitudes within the P-wave portion greater thanthe first threshold amplitude and a number of the one or more maximumamplitudes within the P-wave portion greater than the second thresholdamplitude; determine whether the first sum is equal to a first targetnumber of amplitudes; and determine whether an absolute value of adifference between the first sum and the second sum is less than orequal to a second target number of amplitudes.
 10. The medical devicesystem of claim 4, wherein the processing circuitry is furtherconfigured to: determine a first derivative of the characteristic signalcorresponding to the characteristic window; determine a secondderivative of the characteristic signal corresponding to thecharacteristic window; and determine, based on the first derivative ofthe characteristic signal and the second derivative of thecharacteristic signal, whether the characteristic signal corresponds toa noise event.
 11. The medical device system of claim 10, wherein theprocessing circuitry is configured to: identify a maximum value of thesecond derivative of the characteristic signal within the baselineportion of the characteristic window; and identify a minimum value ofthe second derivative of the characteristic signal within the baselineportion of the characteristic window, wherein the processing circuitryis configured to determine, based on the maximum value and the minimumvalue, whether the characteristic signal corresponds to a noise event.12. A method comprising: sensing, by a medical device comprising two ormore electrodes, a cardiac signal via the two or more electrodes;identifying, by processing circuitry based on the cardiac signal, a setof P-wave windows; determining, by the processing circuitry based on thecardiac signal within each P-wave window of the set of P-wave windows, acharacteristic signal; processing, by the processing circuitry, thecharacteristic signal to identify one or more signal characteristics;determining, by the processing circuitry based on the one or more signalcharacteristics, whether the characteristic signal includes a P-wave;determining, by the processing circuitry based on whether thecharacteristic signal includes a P-wave, whether an atrial arrhythmia isdetected; and generating, for output by the processing circuitry, basedon the characteristic signal including the P-wave, an indication thatthe atrial arrhythmia is detected.
 13. The method of claim 12, whereinmethod further comprises: identifying, by the processing circuitry inthe cardiac signal, one or more pairs of consecutive R-waves;determining, by the processing circuitry for each pair of consecutiveR-waves of the one or more pairs of consecutive R-waves, whether anRR-interval corresponding to the respective pair of consecutive R-wavesis greater than an RR-interval threshold; and identifying, by theprocessing circuitry for each pair of consecutive of R-waves having anRR-interval greater than the RR-interval threshold, a P-wave window ofthe set of P-wave windows.
 14. The method of claim 12, whereindetermining the characteristic signal comprises determining an averageof the cardiac signal within the set of P-wave windows.
 15. The methodof claim 12, wherein the characteristic signal corresponds to acharacteristic window, and wherein method further comprises:determining, by the processing circuitry, a baseline portion of thecharacteristic window; determining, by the processing circuitry, aP-wave portion of the characteristic window, wherein the baselineportion is prior to the P-wave portion, and processing, by theprocessing circuitry, a first portion of the characteristic signalwithin the baseline portion of the characteristic window and a secondportion of the characteristic signal within the P-wave portion of thecharacteristic window to identify the one or more signalcharacteristics.
 16. The method of claim 15, wherein determining whetherthe characteristic signal includes a P-wave comprises: determining asecond derivative of the characteristic signal corresponding to theP-wave portion of the characteristic window; identifying one or moremaximum amplitudes in the second derivative of the characteristic signalcorresponding to the P-wave portion of the characteristic window;comparing each maximum amplitude of the one or more maximum amplitudesto a first threshold; identifying one or more minimum amplitudes in thesecond derivative of the characteristic signal corresponding to theP-wave portion of the characteristic window; comparing each minimumamplitude of the one or more minimum amplitudes to a second threshold;and determining that the characteristic signal does not include a P-wavewhen: a number of the one or more maximum amplitudes greater than orequal to the first threshold is equal to one; or a number of the one ormore minimum amplitudes less than or equal to the second threshold isequal to one.
 17. The method of claim 16, further comprising determiningthat the characteristic signal does not include a P-wave when: thenumber of the one or more maximum amplitudes greater than or equal tothe first threshold is not equal to one; and the number of the one ormore minimum amplitudes less than or equal to the second threshold isnot equal to one.
 18. The method of claim 15, further comprising:determining, by the processing circuitry, a first derivative of thecharacteristic signal corresponding to the characteristic window;determining, by the processing circuitry, a second derivative of thecharacteristic signal corresponding to the characteristic window; anddetermining, by the processing circuitry based on the first derivativeof the characteristic signal and the second derivative of thecharacteristic signal, whether the characteristic signal corresponds toan atrial flutter event.
 19. The method of claim 18, further comprising:identifying, by the processing circuitry, one or more maximum amplitudesof the second derivative of the characteristic signal within thebaseline portion of the characteristic window, wherein each maximumamplitude of the one or more maximum amplitudes is associated with arespective positive going zero crossing of the first derivative of thecharacteristic signal; determining, by the processing circuitry, whethereach maximum amplitude of the one or more maximum amplitudes within thebaseline portion is greater than a first threshold amplitude;identifying, by the processing circuitry, one or more minimum amplitudesof the second derivative of the characteristic signal within thebaseline portion of the characteristic window, wherein each minimumamplitude of the one or more minimum amplitudes is associated with arespective negative going zero crossing of the first derivative of thecharacteristic signal; determining, by the processing circuitry, whethereach minimum amplitude of the one or more minimum amplitudes within thebaseline portion is less than a second threshold amplitude; identifying,by the processing circuitry, one or more maximum amplitudes of thesecond derivative of the characteristic signal within the P-wave portionof the characteristic window, wherein each maximum amplitude of the oneor more maximum amplitudes is associated with a respective positivegoing zero crossing of the first derivative of the characteristicsignal; determining, by the processing circuitry, whether each maximumamplitude of the one or more maximum amplitudes within the baselineportion is greater than the first threshold amplitude; identifying, bythe processing circuitry, one or more minimum amplitudes of the secondderivative of the characteristic signal within the P-wave portion of thecharacteristic window, wherein each minimum amplitude of the one or moreminimum amplitudes is associated with a respective negative going zerocrossing of the first derivative of the characteristic signal; anddetermining, by the processing circuitry, whether each minimum amplitudeof the one or more minimum amplitudes within the P-wave portion is lessthan the second threshold amplitude, wherein determining whether thecharacteristic signal corresponds to an atrial flutter event comprises:calculating a first sum of a number of the one or more maximumamplitudes within the baseline portion greater than the first thresholdamplitude and a number of the one or more maximum amplitudes within thebaseline portion greater than the second threshold amplitude;calculating a second sum of a number of the one or more maximumamplitudes within the P-wave portion greater than the first thresholdamplitude and a number of the one or more maximum amplitudes within theP-wave portion greater than the second threshold amplitude; determiningwhether the first sum is equal to a first target number of amplitudes;and determining whether an absolute value of a difference between thefirst sum and the second sum is less than or equal to a second targetnumber of amplitudes.
 20. A computer-readable medium comprisinginstructions that, when executed by one or more processors, cause theone or more processors to: sense, via two or more electrodes of amedical device, a cardiac signal; identify, based on the cardiac signal,a set of P-wave windows; determine, based on the cardiac signal withineach P-wave window of the set of P-wave windows, a characteristicsignal; process the characteristic signal to identify one or more signalcharacteristics; determine, based on the one or more signalcharacteristics, whether the characteristic signal includes a P-wave;determine, based on whether the characteristic signal includes a P-wave,whether an atrial arrhythmia is detected; and generate, for output,based on the characteristic signal including the P-wave, an indicationthat the atrial arrhythmia is detected.